(a) Field of the Invention
The present invention relates to a method for controlling the thickness distribution of a thin film and, more particularly, to a method suitable for controlling the thickness distribution of an oxide film formed in an in-situ steam generation (ISSG) technique.
(b) Description of the Related Art
The process for manufacturing semiconductor devices generally uses the step of forming a silicon oxide film on a wafer. An ISSG technique is known to form the silicon oxide film, as described in Patent Publications JP-2003-086716A and JP-2002-373984A. The ISSG technique has an advantage over the conventional dry-oxidation technique etc. in that the resultant silicon oxide film has a uniform thickness.
It is known that the silicon oxide film (referred to as merely oxide film hereinafter) formed by the ISSG technique has an in-plane thickness distribution depending on the combinational process condition (process condition) used therein. For example, the in-plane thickness distribution varies depending on the H2 concentration and ambient pressure in the ISSG process. More specifically, a higher H2 concentration provides an oxide film having a larger thickness in the peripheral area than in the central area of the wafer, whereas a lower H2 concentration provides an oxide film having a smaller thickness in the peripheral area than in the central area. A higher ambient pressure provides an oxide film having a larger thickness in the central area than in the peripheral area of the wafer, whereas a lower ambient pressure provides an oxide film having a smaller thickness in the central area than in the peripheral area.
FIG. 9 shows an example of the ambient pressure dependency of the thickness of the oxide film formed by the ISSG process. In this example, the ambient pressure in the ISSG process is varied, with the process temperature being fixed at a specific temperature and the H2 concentration in the ambient gas being fixed at 1%. As understood from FIG. 9, the thickness distribution of the oxide film has a significant dependency on the ambient pressure in the ISSG process.
It may be expected that since the film thickness has the ambient pressure dependency and the H2 concentration dependency of the thickness distribution, an optimum combination of the H2 concentration and the ambient pressure provides a smaller range of variation in the in-plane film thickness. However, it is known that control of the H2 concentration varies the thickness of the oxide film in a larger amount to render the film thickness being out of control.
Thus, in the conventional ISSG technique, the ambient pressure is varied stepwise from 7 Torr to 10 Torr, for example, to form oxide films, with the H2 concentration being fixed for a specific film thickness, and the thickness distribution of the resultant oxide film is measured for each of the ambient pressures, as shown in FIG. 9. Thereafter, a process condition or ambient pressure, which provided an oxide film having an optimum thickness distribution is selected. By employing the selected process condition or ambient pressure, the ISSG process is conducted for forming oxide films in the product semiconductor devices.
In the conventional technique as described above, since the ambient pressure dependency of the in-plane thickness distribution of the oxide film must be investigated for a variety of combinational process conditions each including a processing temperature, a H2 concentration etc., a large amount of fundamental data must be obtained for this investigation. In particular, if maintenance of the oxidation system, such as replacement of the lamp or cleaning of the chamber which affects the thickness distribution, is performed, the fundamental data must be again obtained. Thus, the technique for improving the in-plane thickness distribution to obtain an excellent in-plane thickness uniformity of the oxide film has been required in the ISSG process, and it is an essential issue in the ISSG process to meet this requirement.